1. Technical Field
This invention relates to piston rings for use with pistons of an internal combustion engine and, more particularly, to a seal for a first ceramic matrix composite part in sliding contact with a second part, also made of a ceramic matrix composite material. The seal being commonly known as a piston ring being comprised of an elongated member having an outer contacting edge for contacting the ceramic matrix composite material in sliding contact, the edge of the sealing member having a longitudinal groove formed therein; and, a quantity of an erosion-resistant material tribologically matched to the ceramic contacting surface, such as Tungsten Carbide Cobalt, disposed throughout the length of the groove. The ring can be made as described or coated with a thin, 50 to 500 Angstroms, coating of Titanium Nitride or other similar material to further reduce wear and increase engine performance by reducing the relative friction between the piston ring edge surface and the ceramic matrix composite structure.
2. Background Art
In the basic operation of an internal combustion engine, a piston moving up and down within a cylinder draws in a combustible mixture of fuel and air on its down-stroke and compresses it on its up-stroke. The compressed mixture is then ignited and burns to create energy within the cylinder, which energy is transferred to the piston as power. If the piston is not properly sealed to the cylinder, blow-by occurs and the compression of the mixture is reduced or non-existent thereby reducing or eliminating the effectiveness of that piston/cylinder within the engine. The foregoing description is, of course, quite general and two-cycle, four-cycle, diesel, and rotary engines vary slightly in their mechanics. The basic fact is that the entity creating the compressed volume within an area must have an adequate seal to prevent blow-by or efficiency is lost to some degree. While the principles of the present invention are applicable to a wide variety of engine types employing the special materials to be described, a standard cylindrical piston moving up and down within a cylindrical cylinder of an engine block having one or more cylinders will be used as the embodiment chosen for description hereinafter and in the drawings which accompany it. It should be understood, however, that it is intended the invention described and claimed herein be accorded a breadth in keeping with the scope and spirit of the disclosure as applied to all engine types of applicability.
In a classic prior art engine, the cylinders and pistons are of metal. Early engines were of cast iron while later engines are of lighter metals and alloys of, for example, aluminum. Also, the use of cylindrical sleeve inserts for the walls of the cylinders has become popular. Regardless of the materials used for the cylinder walls, the standard method of sealing the space between the cylinder walls and the pistons is the piston ring. A space exists (and must exist) because of dissimilar thermal expansion of the metal components. If the piston and cylinder were sized exactly with only enough clearance for the piston to fit within the cylinder with a coating of lubricating oil film between them, as soon as the metal heated from the combustion within the cylinders and expanded, the piston would seize within the cylinder. If enough clearance was provided initially to allow for expansion, the blow-by would be so extreme that the engine would not run sufficiently to get up to temperature and create the proper seal. Thus, the expansion space is provided and the clearance gap is closed with piston rings as shown in FIGS. 1 and 2. There may be one ring 10, two rings 10, or more, depending on the engine design and the objectives thereof. Regardless of the number, each piston ring 10 is disposed in a ring groove in the peripheral surface of the piston adjacent the top thereof. The ring 10 is not a complete circle. Rather, it is incomplete and has a gap 11 at its ends so that it can be compressed in diameter. The ring groove in the piston is made deep enough that the ring 10 can move in and out as the piston expands and contracts.
Early low compression engines with piston rings made of the materials available at the time tended to form carbon within the ring grooves over time. Also, one had to take great care in breaking-in the engine when new to allow the piston rings to properly "seat" to the cylinder walls. As a result, it was not uncommon to have to do a ring job, i.e. replace the piston rings with new ones, at mileages as low as 50,000 miles. Recent engine designs and the materials employed for the pistons, cylinders, and rings as well as the modern more accurately computer-controlled engines which have less tendency to form carbon in the ring grooves have pushed the life expectancy to the point that the average automobile owner never has to even think about a ring job within the useful life of his/her automobile.
While engine designs and materials have certainly improved over the years, there still remain deficiencies such as lower than desirable fuel efficiency and higher than desirable pollution emissions. In a co-pending application entitled HIGH-EFFICIENCY, LOW-POLLUTION ENGINE, Ser. No. 08/515,608 by the inventors herein filed on Aug. 16, 1995 and assigned to the common assignee of this application, an improved structural fiber reinforced ceramic matrix composite (FRCMC) material is disclosed having high breakage resistance and particular applicability to use for parts in a high temperature internal combustion engine, the teachings of which are incorporated herein by reference. Being of the FRCMC material, the pistons and cylinders can withstand much higher operating temperatures than conventional internal combustion engines. Moreover, since the coefficient of thermal expansion of the parts is much lower, much closer tolerances can be maintained without the danger of engine seizure. Still, however, to operate at as high a compression ratio as possible without efficiency-robbing blow-by, there must be piston rings. Conventional piston rings are not adequate, however. Despite the many improvements in materials and wear resistance, prior art piston rings are intended for use in metal engines. The FRCMC material, in conjunction with its ceramic wear coatings will quickly erode even the hardest conventional metallic piston rings.
Wherefore, it is an object of the present invention to provide a piston ring that can be used in a ceramic cylinder with a ceramic piston without rapid erosion.
It is another object of the present invention to provide a piston ring that can be used in a FRCMC cylinder in combination with a FRCMC piston without rapid erosion.
It is still another object of the present invention to provide a piston/rotor seal that can be used in an internal combustion engine of any type without rapid erosion.
Other objects and benefits of this invention will become apparent from the description which follows hereinafter when read in conjunction with the drawing figures which accompany it.